Conventionally, in analog television broadcasting, in a case of receiving broadcast waves in a weak electric field, reduction in a receiving level by several dB greatly degrades the quality of pictures, or in a case of receiving broadcast waves in an urban area, unpleasant ghost pictures are generated due to waves which are reflected from buildings. Therefore, an antenna which has a high gain in a direction of incoming radio waves and a low gain in a direction of reflected radio waves must be placed in a location where the incoming radio waves are as strong as possible. Thus, as a conventional antenna mount method, there has exclusively been adopted a method of supporting an antenna that has a directivity in the horizontal direction using a metallic pole, to mount the same in a high position on the roof.
On the transmitting end that transmits broadcast waves of the analog television broadcasting, horizontal polarization has been adopted as polarization of the broadcast waves. This is because the reduction in the receiving level resulting from a disturbance of the received electric field caused by a current that is induced on the metallic pole of the antenna becomes smaller when the antenna (receiving end) receives the horizontally polarized waves, and further, on the transmitting end, a transmission antenna that has the horizontal polarization and no directivity in the horizontal direction is realized.
Conventionally, a current inducing type dipole antenna has been exclusively employed as the antenna on the receiving end of the analog television broadcast waves because it has a small resistance to winds, has a large equivalent receiving area, i.e., has a wide receiving band, and further can increase the gain by easily increasing the number of elements.
It is also possible to receive the broadcast waves of the analog television broadcasting not using the above-mentioned outdoor antenna but using an indoor antenna which does not need an antenna line from the wall to the receiver. Also as such indoor antenna, a current inducing type dipole antenna has been conventionally employed exclusively, because it has a wide receiving band, and it can be realized in a simple structure and at low cost (for example, refer to Laid-open Japanese utility model publication No. Hei. 05-80014 (p. 2, FIG. 1)).
On the other hand, in digital television broadcasting which has recently become popular, when broadcast waves of a relatively strong electric field are received in an urban area, no ghost picture occur in principle even when there are reflected waves from the buildings, in contrast to the analog television broadcasting. Therefore, attention is being given to the usability of the above-mentioned indoor antenna which does not need an antenna line from the wall, as an antenna for receiving broadcast waves of the digital television broadcasting.
Also on the user side, there is a demand that broadcast waves of the digital television broadcasting are received using an indoor antenna also in the case of receiving broadcast waves in a weak electric field, and its realization has been expected more than in the analog television broadcasting, because it has previously been widely known that the digital television broadcasting has a feature that the picture quality is not deteriorated unless the receiving level of the radio waves becomes lower than a threshold value. Further it has an advantage of freely placing a receiver indoor when using the above-mentioned indoor antenna.
When supposing that the digital television broadcast receiving antenna is realized by an indoor antenna, an antenna which has a directivity in a specific direction and can change the directivity to a direction of incoming radio waves by an electronic control is demanded, because the digital television broadcast receiving indoor antenna has also a physical merit of not wasting the gain.
Further, since it is considered that broadcasting of the digital television broadcasts with horizontal polarization is suited for receiving the radio waves even by an analog television broadcast receiving antenna which has already become widely available, an antenna that can receive horizontally polarized waves is suitable for the digital television broadcast receiving indoor antenna.
In light of the foregoing, an antenna utilizing a magnetic current that is induced at an aperture which is provided on a metallic plate or a metallic box, as a radiation source (hereinafter, referred to as a magnetic current inducing type antenna) can be placed in a smaller area than a current inducing type antenna that has conventionally been used as the indoor antenna because this antenna can receive the horizontally polarized waves in a vertically long slender shape. Furthermore, there is no need of orienting the antenna toward a direction of incoming radio waves because it has almost no horizontal directivity. When noticing these characteristics, this magnetic current inducing type antenna is promising as a unit antenna element for the digital television broadcast receiving indoor antenna which can respond the need for the above-mentioned digital television broadcast receiving antenna. (For example, refer to Japanese Published Patent Application No. Sho. 58-15303 (p. 7, FIG. 8) and Japanese Published Patent Application No. 2003-124738 (p. 6, FIGS. 1–3)).
As described above, the magnetic current inducing type antenna that is considered as promising as a digital television broadcast receiving antenna is considered as suitable for the digital television broadcast receiving antenna, but a digital television broadcast receiving antenna employing such magnetic current inducing type antenna has not been realized yet.
The main reason is that, like the current inducing type dipole antenna, the unit antenna element of the magnetic current inducing type has a high Q value indicating the strength of the resonance, and cannot receive broadcast waves in a wide band that is expected in the digital television broadcasting, for example broadcast waves over a wide band extending from 470 MHz to 710 MHz in Japan.
That is, in order to receive the broadcast waves over a wide range extending from 470 MHz to 710 MHz or the like, there is no choice of either combining plural unit antenna elements having different resonance frequencies, or lowering a Q value of a unit antenna element and electronically tuning the unit antenna element for the broadcast waves. However, in the former case, the antenna becomes larger than the current inducing type dipole antenna and is not practical to use, while in the latter case, a reactance changing range that is required by a tuning element which is provided in the unit antenna element becomes large and it is difficult to realize.
Further, as for a digital television broadcast receiver, there has been no measure for realizing a receiver which can orient the antenna directivity toward a direction of incoming broadcast waves by an electronic control, and in which there is no antenna part jutting while an antenna is mounted or integrated therein.